This invention relates to improved apparatus for coating a substrate with molten fluent material and in particular relates to apparatus referred to in the context of this application and the appended claims as thermal spray apparatus for producing a ray or stream of molten fluent material with which the substrate is sprayed and which material upon hardening provides the substrate with a coating of the material.
It will be understood by those skilled in the art that as used in the context of this specification and the appended claims, the term "ray" or "stream" is used in the sense of a stream of particles traveling in the same line, the expression "heat fusible fluent material" is used to mean powdered or flowable thermoplastic and thermosetting material such as PTFE, e.g. Teflon, .RTM. epoxies, polyester, polyurethane, polyvinylchloride, polyethylene and the like, the expression "molten fluent material" is used in the sense of being heated to its melting point prior to or simultaneously with striking the substrate, the term "substrate" is used to mean the surface of the object to be coated, and the term "flame" is used to means the result of combustion of an inflammable gas or a stream of gas undergoing combustion and the term "flame" in the context of this specification and the appended claims is also used to mean the arc struck between two electrodes between which an ionizable gas passes to effect or produce a plasma.
As known to those skilled in the art of coating substrates with fluent material, there is a great need for improved coating apparatus for coating large and stationary structures or substrates such as metal tanks, large construction substrates such as the roof of a tunnel, pipeline supports, pipelines, and other structures or objects that are too large to be coated by the conventional oven coating method wherein the object or structure must be sufficiently small to permit being placed inside an oven for pre-heating and whereafter the object of structure is coated with fluent material such as plastic and then re-inserted into the oven for post-curing. Presently, as is known, there is a great need for both initial coating and maintenance coating of such large objects or structures but the inconvenience and excessive cost of dismantling such large objects to permit insertion and re-insertion into an oven virtually prohibits oven coating of such large objects.
As is further known to those skilled in the art, wooden, cloth and paper substrates cannot be subjected to coating with fluent materials such as the above-noted plastic materials in the conventional oven heating method because these products or substrates deteriorate and present such a outgassing problem that oven coating would be rendered virtually useless. As is still further known to those skilled in the art, piece parts such as glass bottles, tin cans, or food packaging, which are required to be coated in large numbers per unit of time, cannot be coated in a cost-effective manner in the above-noted conventional oven heating method.
The use of the conventional "fluidized bed" coating method is, of course, known to the art, but such coating method requires that the substrates be heated to melt the fluent materials applied thereto and this preheating requirement has the attendant temperature deterioration and outgassing problem noted above, particularly with regard to wooden, cloth or paper substrates. In addition, the coating of the above-noted articles at large numbers per unit of time by the fluidized bed method has the intrinsic problem of article handling which is both time consuming and expensive whether done manually or by automation.
The "electrostatic spray" coating method is also known to the art and may be employed either with or without pre-heating of the substrate since the electrostatic charge holds the coating material on the substrate until it is used and, in the case of thermosetting material, the coating with the electrostatic spray obviously requires the substrate to be post-cured. Also, electrostatic spray apparatus is expensive, not readily portable, and does not lend itself to coating of the large substrates noted above.
The concept of "flame spraying" or "hot spray" has existed for some time as an alternative to circumvent the problems noted above with regard to the prior art coating methods and apparatus and has been conceived as a method wherein the heating source quickly, for example in a second or less, melts the fluent material such as one of the above-noted plastic materials and maintains the material in a molten state until applied to the substrate where the material will harden immediately but yet will remain in a plastic state sufficiently long to provide a homogeneous film or coating. The major advantage of such flame spraying or hot spray coating is that the substrate is subjected to very little heat whereby the above-noted problems with regard to substrate deterioration and outgassing it overcome. Further, such flame spraying is readily suitable for coating large objects or structures of the type noted above as they exist as there would be no dismantling or disassembly requirement for coating.
At present, at least insofar as is known, two approaches to flame spraying or hot spray coating have been used. One utilizes a heat source similar to a blow torch or welding torch and uses acetylene and oxygen as fuel. The limited success of this coating method is generally attributed to the prior art problem or difficulties of introducing finely ground plastic or powder, heat fusible fluent material, directly into, or at least near, the open flame to produce a stream of molten fluent material without causing combustion or oxidation of the material or reaction (i.e. chemical reaction) between the flame and material, which greatly reduce the integrity or homogeneity of the coating or film applied to a substrate. This prior art problem is typified by the handheld thermal spray or flame spraying apparatus 110 shown diagrammatrically in FIG. 9 which produces a stream of molten fluent material illustrated collectively by dashed lines 112 by introducing a stream or streams of heat fusible fluent material indicated by dashed lines 114 and 116 into a stream of heated gas indicated diagrammatrically by dashed line 118, which stream of heated gas is produced by the combustion or burning with an open flame, indicated diagrammatically at 120 and 120A, in a burner 132 of a stream of air and inflammable gas indicated by dashed line 134. Since there is no barrier intermediate the flame 120 and 120A and the heat fusible fluent material, reaction, i.e. chemical reaction, can occur between the flame and material causing the above-noted prior art problem.
The other flame spraying or hot spray coating method is referred to in the art as "plasma spraying," and is typified by the flame spraying apparatus and method disclosed in U.S. Pat. No. 3,935,418 issued Jan. 12, 1976 to Mille Stand et al. The advantage of this plasma method over the other method noted above (inflammable gas) is that a reducing atmosphere is created and consequently less coating material is oxidized. The appearance of the plasma can be compared to a high heat cutting torch such as the above-noted acetylene cutting torch where, in order to be effective, the fluent material must be introduced directly into the plasma, or at the border thereof, and hence into a least close proximity to the arc (flame) in order to melt the material into a molten state for coating. This prior art problem is typified by the hand-held plasma thermal spray apparatus or device 210 shown diagrammatically in FIG. 10 which produces a stream of molten fluent material illustrated collectively by dashed lines 212 by introducing a stream or streams of heat fusible fluent material indicated by dashed lines 214 and 216 into a stream of heated gas (plasma) indicated diagrammatically by dashed line 218 which stream of heated gas (plasma) is produced or effected by passing a stream, or streams, 234 and 234A of ionizable gas between two electrodes, anode 236 and cathode 238, to produce or effect the plasma 218. Again, since there is no barrier intermediate the arc (flame) 220 and 220A, and the heat fusible fluent material 214-216, and the stream of molten fluent material 212, reaction, i.e. chemical reaction, can occur between the arc (flame) and material causing the aforenoted prior art problems of material combustion, oxidation, etc.
Accordingly, there exists a need in the coating art of new and improved coating apparatus which overcomes the above-noted problems attendant to the noted prior art coating apparatus and in particular a new and improved invention which solves the problem of reaction between the flame and material, material combustion and oxidation associated with the prior art requirement of introducing the fluent material directly, or nearly directly, into the flame. Further, there exists a need for new and improved coating apparatus which is relatively inexpensive, readily portable, whereby it may be easily and inexpensively moved from one location to another and in particular be used to coat large objects and structures of the type noted above without requiring their dismantling or disassembly and which may also be readily used to coat articles in large number per unit of time of the type also noted above.